Use of a hot mirror to improve the efficiency of a lamp is well known. See, for example U.S. Pat. No. 4,229,066, by Rancourt et al. which discloses a design for such a hot mirror coating. In U.S. Pat. No. 4,663,557 Martin and Rancourt teach the use of a coating comprised of alternating layers of two materials, SiO.sub.2 as a low index material, and Ta.sub.2 O.sub.5 as a high index material to achieve a coating suitable for operation at temperatures in excess of 500.degree. C. Their coating was fabricated by a technique known as e-beam evaporation. The maximum operating temperature of their coating in air is not clear from their specification; however, as is indicated by Kuus in the patent cited below, recrystallization of Ta.sub.2 O.sub.5 occurs above 800.degree. C., causing grains to form in the coating. The grains remain in the coating after exposure to high temperature, causing undesirable light scattering in subsequent operation. Martin and Rancourt state that their coating changes so as to become scattering to visible light when baked in air at 1100.degree. C. for a number of hours.
In U.S. Pat. No. 4,734,614, Kuus discloses a lamp design in which the lamp envelope has a heat resistant interference coating on its inner or on its outer surface. The coating is comprised of alternating layers of Nb.sub.2 O.sub.5 and SiO.sub.2. Kuus claims that his coating is superior to the Ta.sub.2 O.sub.5 /SiO.sub.2 layers of the prior art such as are specified in the Martin and Rancourt patent, the superiority being due to their greater stability at temperatures of 800.degree. C. or more. He points out that near 800.degree. C. Ta.sub.2 O.sub.5 crystallizes to form .beta.-Ta.sub.2 O.sub.5, causing undesirable scattering of light by the coating. Other disadvantages cited by Kuus are a tendency of the coating with Ta.sub.2 O.sub.5 to crack during operation of a lamp to which it has been applied and loss of transmission of the coating known as graying which may occur, especially under conditions where little oxygen is present. That the niobia/silica combination is not suitable for coatings which must operate at temperatures of 1000.degree. C. has been demonstrated in tests performed by us in which Nb.sub.2 O.sub.6 /SiO.sub.2 optical interference coatings were baked at 1000.degree. C. for an hour or more. After baking the coatings displayed excessive scattering of visible light appearing milky white under visual inspection.
U.S. Pat. No. 4,949,005 by Parham et al, pertains to the fabrication of interference coatings consisting of more than 12 alternating layers of Ta.sub.2 O.sub.6 and SiO.sub.2 by processes known to the art as chemical vapor deposition (CVD) or low pressure chemical vapor deposition (LPCVD). These processes allow uniform coatings to be deposited on substrates that are non planar, such as either the inner or outer surfaces of a cylindrical lamp burner envelope. The term "lamp burner" is taken to mean that part of the lamp within which light is generated. The Parham patent teaches the use of a prescribed heat treating process which causes the deposited film to crack in such a manner as to relieve the stress in the film while maintaining acceptable adhesion and spectral performance. After heat treatment, coated parts function without degradation after repeated cycling between room temperature and 900.degree. C.
High temperature coatings which are fabrication by sputtering materials previously cited are also known to the art. Such coatings can be cycled between room temperature and 900.degree. C. without degradation, and are suitable for use on the outside of tungsten halogen lamps.
The prior art coatings such as those heretofore discussed can function at or below temperatures (800.degree. to 900.degree. C.) that are normally reached on the outside of a halogen lamp envelope during operation. At higher temperatures, these coatings develop excessive scatter and suffer degradation of their optical properties. In some applications coatings may be required to function at higher temperatures. For example, the temperature on the outer surface of a mercury discharge lamp burner may reach a temperature in excess of 1000.degree. C. It is often desirable to provide an interference filter on this outer surface for the purpose of selectively reflecting or transmitting portions of the electromagnetic spectrum. The coating may have the function of selectively directing the light from the burner to some region of space outside the lamp, or of reflecting a portion of the spectrum (for example, infrared energy) back into the lamp while transmitting another portion, or a combination of these functions.
It is therefore an object of this invention to provide an interference coating which is not degraded by repeated cycles between room temperature and a second temperature, said second temperature being in excess of 1000.degree. C. and as high as 1200.degree. C., said coating being able to function before and after cycling as described in the paragraph above.
It is a further objective of this invention to provide an alternative coating to those coatings already available to the prior art, said alternative coating functioning as a hot mirror, cold mirror, or other type of coating that can be successfully employed on the outer surface of a device such as a halogen lamp, where this outer surface reaches a temperature in excess of 500.degree. C. and as high as 1200.degree. C.